A fluid suspension is often used for realizing sophisticated suspension control in concatenation with an electronic control device because it is comparatively easier to change the suspension characteristics.
Such control usually includes adjustment of a vehicle body height, which is increased a desired amount by supplying a predetermined amount of fluid into a fluid suspension chamber of each wheel from a fluid source, while it is lowered a desired amount by removing a predetermined amount of fluid from the fluid chamber.
An example of structure and function for adjustment of body height by such a fluid suspension is disclosed in the Japanese Laid-open Patent No. 119623/1985. The vehicle fluid suspension circuit of the prior art usually employs the structure indicated in this patent, comprising fluid sources provided to the front and rear sides of vehicle, a suspension system having a fluid chamber provided to each wheel, a pipe connecting the fluid sources with each fluid chamber, and valves provided immediately before the fluid chamber of each pipe.
Adjustment of body height is intended to set the body height at a desired value and also to keep the body horizontal. If the fluid chamber valves of respective wheels are opened simultaneously as in the circuit structure of the prior art, in order to simultaneously supply fluid from a fluid source into the fluid chambers in the suspension system of each wheel in order to realize adjustment of body height as quickly as possible, the fluid chambers are also opened to each other. Since the respective loads applied on each wheel are generally not equal, the attitude of vehicle is unbalanced.
Therefore, integrated adjustment of a vehicle's body height while keeping the vehicle horizontal inevitably involves sequentially adjusting all wheels by varying the time during when each wheel's fluid chamber valve is opened, connecting only one fluid chamber at a time to the fluid source, and gradually increasing (or lowering) the body height at each wheel. However, this method results in a problem that a longer time is required to adjust body height.
On the other hand, there is no large difference of load distribution between the right and left sides of the vehicle compared to that between the front and rear sides, depending on the location of the engine. This suggests a method to adjust body height by dividing the circuit into two systems--one for the front wheels and one for the rear wheels. Namely, adjustment of body height is carried out separately for the front and rear wheels by providing a valve in the circuit path between the front wheels and the rear wheels side. This method has an advantage that the adjustment time is halved.
However, this method still contains the problems that the body height adjusting speed is lower than that of the simultaneous body height adjustment for all wheels, and that structure and control are complicated by the valve and the total weight is more.